1. Field of the Invention
This invention relates to improvements in methods and circuits for driving DC brushless, polyphase motors, such as a spindle motor of a hard disk drive, or the like, and more particularly to improvements in such driving methods and circuits that at least reduce the acoustic noise in motors of this type, and to improvements in driving methods and circuits for driving such motors using driving voltages that are approximations of waveforms constructed from substantially sinusoidal waveform segments, and, more particularly, constructed from substantially linear approximations of such waveform approximations.
2. Relevent Background
In the operation of DC brushless, polyphase motors of the type to which the invention pertains, efficient motor drive requires that the excitation current applied to the motor phases be aligned with the bemf generated by individual phases. One of the best schemes for achieving this alignment is the use of a phase-locked loop (PLL), which adjusts the phase and frequency of the commutation, so that the bemf of the undriven winding passes through zero in the center of the appropriate commutation state. This scheme works well when the shape of the commutation waveforms includes an undriven region, as in a conventional 6-state, +1, +1, 0, -1, -1, 0, sequence.
Since the +1, +1, 0, -1, -1, 0 sequence has sharp transitions between driving states, this sequence has many high frequency components. These tend to excite mechanical resonances in the motor, which results in the creation of undesirable acoustic noise. Moreover, the step-function tristating of the undriven motor phases, together with the step-function driving waveform itself produces a degree of torque ripple in the motor. The torque ripple results in an unevenness or jerkiness in the motor rotation, which also excites resonances in the motor, also causing undesirable acoustic noise.
Thus, if it is desired to reduce acoustic noise, a sine wave shaped excitation signal is more appropriate than the 6-state sequence. If the motor driver consists of sinusoidal current sources, the same voltage sensing PLL described above can be used. However, when the duty cycle of the driver is varied sinusoidally, the motor driver excitation is pulse-width modulated (PWM) to minimize power dissipation in the driver IC. This permits lower cost packaging and an overall saving in system cost. However, in the past, it has been difficult to generate currents that have a pure sinusoidal waveform, particularly when the currents are relatively high, and when a PWM scheme is desired to be used.
In previous application Ser. No. 09/300,754, after initial baseline cancellation, a driving waveform was formed of concatenated segments of 120.degree. of zero, followed by 120.degree. of "up hook", and 120.degree. of "down hook". The up-hook and down-hook waveforms were generated in two MDACS. The operation of the MDACs had some problems that resulted in difficulty in forming the desired driving waveforms. In particular, circuit tradeoffs needed to be made so that the resulting waveforms could properly operate in the polyphase DC motor environment.
Also, using an MDAC to form waveforms segments that are themselves formed from sinusoidal waveforms segment combinations requires special MDAC design and operational considerations. A design in which the driving waveforms can be constructed from linear waveform segments to approximate the sinusoidal waveforms segment combination would significantly ease the realization and operation of such MDAC.
Consequently, what is needed is a disk drive and method for operating it in which the noise associated with the drive in operation is reduced or eliminated. What is additionally needed is a disk drive and method that employs drive signals that are developed from sinusoidal signal segments, which can be easily approximated and generated by concatenated linear signal segments.